The proposed research is of extremely high significance for public health. Overweight and obesity affect the majority of US adults and a growing proportion of children and youth. The causes are not well understood, but are believed by most scientists to be largely due to a food environment that promotes excess energy intake. Large food portion sizes are thought to play a significant role in overeating, and are pervasive across virtually every setting in which people live, work and socialize. Highly controlled laboratory studies have provided strong evidence that large portion sizes have short-term effects to increase energy intake, but have not investigated the longer-term effects of large portion sizes on energy intake or body weight change. Small portion sizes have not been evaluated for their potential protective effects on energy intake or body weight change. The proposed research bridges a crucial gap from the laboratory to the naturalistic environment by creating a novel model experimental paradigm through which to test the effects of portion sizes in free-living people in the natural environment. The research addresses a critical public health question of whether the pervasive large portion sizes are contributing to excess weight gain among free-living people and are therefore a potent public health risk exposure for overweight, obesity and excess weight gain. No data currently exist that directly address this important public health question. The proposed study aims to examine the effects of large and small portion sizes on body weight over a six-month period among a free-living sample of 200 adults. Change in energy intake is a secondary outcome. Potential moderators and mediators of the association between portion size and body weight change will be examined in an exploratory manner. Participants will be randomized to one of four study conditions for a six-month period: Lunch meal portion size 400 kcal, 800 kcal or 1600 kcal or a no-intervention control group. Participants in the portion size lunch groups will be provided with one lunch meal daily 5 days per week for the six-month period. Body weight and energy intake will be measured at baseline, one, three and six months. It is hypothesized that participants will gain weight in the 1600 kcal portion size condition relative to the 800 kcal portion size condition and relative to the control group.